The progress has astonishingly been made, in the technology relating to laser exposure and development of lithographic printing. Especially, it can be noted that a high power and compact device has been available, without difficulties, for the solid laser and the semiconductor laser each of which has an emitting region in the range of from near-infrared to infrared. These lasers are extremely useful as an exposure source, for making a plate directly from digital data of a computer and the like. Accordingly, it is extremely important to develop a lithographic printing plate precursor that properly copes with this type of the plate making.
The recording layer of a positive type lithographic printing plate precursor for infrared laser contains, as essential components, an alkali-soluble binder resin and an IR dye or the like that absorbs light to generate heat. This IR dye or the like in the unexposed area (image area) acts as a development inhibitor that substantially reduces solubility with respect to a developing solution for the binder resin due to interaction with the binder resin. Meanwhile, in the exposed area (non-image area), an interaction between the IR dye or the like and the binder resin is weakened due to the generated heat, and the binder resin is dissolved in an alkali developing solution to form a lithographic printing plate.
The basic performance required for the lithographic printing plate precursor includes developability that rapidly realizes formation of a vivid image area, and in contrast, chemical resistance by which the recording layer in the unexposed area is accurately retained without being dissolved, and improvement in plate durability that increases print capacity. However, these performances are generally conflicting. Accordingly, it is extremely difficult to improve all the performances at the same time. For example, in JP-A-2007-017913 (“JP-A” means an unexamined publication of Japanese patent application), Japanese Patent No. 4579639 and JP-T-2010-532488 (“JP-T”means published Japanese translation of PCT application), incorporation of a particular polymer into a recording layer of the printing plate precursor disclosed therein has been proposed.
Normally, when a polymer having a large molecular weight is incorporated in a recording layer, improvement in plate durability and chemical resistance in accordance with it can be expected. However, that tends to reduce developability. As a result, it is difficult to improve all the performances with a balanced manner.